Howdy,
TL;DR: go here and read about how to fix this issue:
Ford EATC Repair - O Ring Replacement Tutorial
I had the same symptoms as the original poster:
-Tendency for the MAX AC and AC to cut over to defroster and floor under acceleration.
-Sometimes would refuse to even start blowing AC through the vents, would only do Floor/Defrost.
My vehicle:
2000 Expedition Eddie Bauer, 4.6L but this actually applies to a huge range of Ford, Lincoln and Mercury vehicles.
I had a shop diagnose my AC issues, they said it was low on freon, had a bad check valve and repaired it. They gave it back to me saying it was fixed only to have it STILL cut to defrost from AC! They diagnosed some more, check vacuum, etc, finally said it was the electronics in the head unit and wanted $400 or 500 for a USED replacement. I said nope. Limped it along.
THEN I found a link to a repair for what seems to be a common problem with Ford EATC (electronic automatic temp control, IIRC) units. There are four little solenoids inside the head unit which control the flow of vacuum through the system which are actuated by the electronics. The solenoids have a little o-ring which apparently was undersized and which eventually can become dry and brittle and prevent vacuum from sealing. As a heads up, mine required prying and bending bits of folded metal to get the solenoid plungers out. I didn't think it would function at all based on how much mangling I was doing.
But it worked! I've got solid, cold MAX AC blowing through my vents, even under heavy acceleration!
my local O'reillys had a tray of assorted o-rings with the needed 007 oring that I needed.
National O-Rings 007 - O-Ring | O'Reilly Auto Parts
Good luck!
BTW, I've read a lot of recommendations to check the vacuum lines around the battery and that is probably a good idea but this o-ring issue is apparently very common...
